Just before eye movement execution: processing of visual objects

Abstract

Our visual system is fovea-heavy, which means that in-depth processing occurs only in the centre of the retina, forcing the eyes to make constant movements in order to bring visual elements into focus. Before an eye movement attention shifts to the location where the eyes will land and visual elements presented there are preferentially processed. In the current thesis, we are concerned with how the visual elements present in the environment shape the allocation of attention before eye movements. We first aimed to determine whether pre-saccadic shifts of attention are a precondition of all saccades, irrespective of goals. We showed that whether the saccade was goal-directed, to the intended target, or involuntary, erroneously directed to a capturing distractor, made little difference to the pre-saccadic shift of attention. Retinal displacement caused by involuntary saccades was also accounted for by the visual system. Next, the project investigated how the decision to make an eye movement is affected by the number of target alternatives. We saw evidence that a larger set-size can reduce saccadic reaction times without increasing the error rate, a finding not predicted by a popular model. Further, whether the presence of visual elements in and around the saccade landing point influences the shifts of attention was investigated. We demonstrate that that perceptual sensitivity is greatly influenced by the presence of visual objects, when these are displayed simultaneously with attentional probes. Visual objects alter perceptual sensitivity across space, which is the very variable that is being tested. Finally, we look at the spatial and temporal distribution of visual attention when a saccade target is removed shortly before the eye movement.